Thermal treatment of glass in a fluidized bed

ABSTRACT

The thermal treatment of glass is effected by heat transfer between the glass and a gas-fluidized particulate material which is in a quiescent uniformly expanded state of particulate fluidization. Glass sheets are thermally toughened by immersing each glass sheet, which is at a temperature above its strain point, in the gas-fluidized bed prior to the immersion of the glass in the bed.

This is a continuation of application Ser. No. 717,171, filed Aug. 24,1976, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the thermal treatment of glass, and moreespecially to the thermal toughening of glass articles, for example flatglass or bent glass sheets. Such thermally toughened glass sheets may befor use singly as a motor vehicle windscreen, or as part of a laminatedmotor vehicle windscreen, a side light or rear light for a motorvehicle, or for use in the construction of windscreen assemblies foraircraft and railway locomotives, or in the construction of windows forships, or for architectural uses. Other glass articles such as pressedor blown glass articles may be thermally toughened by the method of theinvention.

2. Description of the Prior Art

The ultimate tensile strength of a glass article can be increased by athermal toughening process in which the glass is heated to a temperatureapproaching its softening point, followed by rapid chilling of the glasssurfaces to induce centre-to-surface temperature gradients through thethickness of the glass. These temperature gradients are maintained asthe glass is cooled through its strain point. This results incompressive stress in the surface layers of the glass sheet withcompensating tensile stress in the central core of the thickness of theglass sheet.

Usually this thermal toughening process is carried out using chillingair directed uniformly at both surfaces of the glass sheet out it isdifficult to obtain a high degree of toughening using air flows,particularly when toughening glass sheets of 3 mm thickness or less.Attempts to increase the degree of toughening of a glass sheet byincreasing the rate of flow of cooling air can give rise to loss ofoptical quality of the surfaces of the glass and distortion of the shapeof the glass sheet due to the buffeting action of the chilling air.

In another thermal toughening process a glass sheet at a temperaturenear to its softening point is quenched in a chilling liquid. Highstresses can be produced by this method. The glass sheets have to becleaned after quenching.

Thermal toughening of a glass sheet has also been proposed by a methodin which a hot glass sheet is immersed in what, in practice, was afreely-bubbling fluidised bed of solid particles, for example sand.

Such a process has not been brought into commercial use hitherto.

The major problem which we have found when attempting to operate such abed for the thermal toughening of glass is the high incidence offracture of the glass sheets during their treatment in the fluidisedbed. The fracture of a glass sheet while being quenched in afreely-bubbling fluidised bed is thought to be caused by the inductionof destructive tensile stresses in the leading edge of the glass sheetdue to nonuniform cooling as the leading edge enters the bed ofparticles in a state of bubbling or aggregative fluidisation.

Loss of glass sheets due to fracture is particularly serious whenattempting to toughen thin sheets of glass, for example of thicknessfrom 2.3 mm to 4.0 mm, to a high stress value, and has been such as torender the process unacceptable for the commercial production oftoughened glass sheets for use in car windscreens for example. Theproblem of fracture also arises to a lesser but still commerciallysignificant extent when seeking to toughen thicker sheets, for exampleup to 8 mm thick.

A freely-bubbling bed in a state of aggregative fluidisation has alsofound to damage hot glass sheets immersed in it. This is due to theirregular forces to which the glass is subjected in a freely-bubblingbed. This can give rise both to changes of overall shape and to morelocalised surface damage, the former occurring particularly with thinnerglass sheets such as those of 2 mm to 3 mm thickness. Such damage aschanges of shape may give rise to difficulties in lamination, andsurface damage may give rise to unacceptable optical quality when thesheet is used as a window or as a component of a laminated window.

The present invention is based on the discovery that the use of agas-fluidised bed in a quiescent uniformly expanded state of particulatefluidisation unexpectedly produces adequate stresses in glass sheetsquenched in it and substantially reduces loss of glass sheets due tofracture in the bed or to change of shape or surface damage so that asuccessful commercial yield is achieved.

SUMMARY

The invention provides a method of thermally treating glass in which theglass is contacted with a gas-fluidised particulate material which is ina quiescent uniformly expanded state of particulate fluidisation. Heattransfer between the surfaces of the glass and the fluidised materialaffects the thermal treatment, for example for thermally toughening orheating the glass.

The glass may be heated to a temperature above its strain point and thenimmersed in a gas-fluidised bed of particulate material which prior tosaid immersion is in a quiescent uniformly expanded state of particulatefluidisation.

The invention is particularly concerned with a method of thermallytoughening a glass sheet comprising heating the glass sheet and thenlowering the hot glass sheet into the quiescent uniformly expanded bedof particulate material. Preferably the bed is maintained at atemperature in the range 30° C. and 150° C. This temperature is selectedin dependence on the fluidisation characteristics of the particles andthe required level of stress in the toughened sheets.

The fluidised bed of particulate material in a quiescent uniformlyexpanded state of particulate fluidisation, which is employed incarrying out the invention, can be defined in terms of the velocity ofgas flow through the bed and the expanded height of the bed. Thequiescent uniformly expanded state of particulate fluidisation existsbetween a lower limit of gas velocity at incipient fluidisation, that isthe velocity at which the particles just become suspended in theuniformly distributed upwardly flowing gas, and an upper limit of gasvelocity at which maximum expansion of the bed occurs while maintaininga free surface at the top of the bed.

The upper limit of fluidisation gas velocity may exceed by a smallamount the velocity at which the first clearly recognisable bubble, forexample 5 mm in diameter, is seen to break the calm surface of the bed.One or two such bubbles may be visible at that gas velocity.

A higher gas velocity results in the development of extensive bubblingin the bed and at the onset of such bubbling there is partial collapseof the bed height.

We believe that by quenching the sheet in a gas-fluidised bed which isin a quiescent uniformly expanded state of particulate fluidisation, anytransient tensile stresses induced in the leading edge of the glasssheet on entry into the fluidised bed are not so severe as to endangerthe glass sheet and to cause it to fracture.

Also the substantially bubble-free nature of the bed ensures that thehot glass is not subjected to irregular forces such as could also giverise to fracture, or to changes in shape of the glass sheet duringquenching, or to surface damage.

Previously it has been thought that, to obtain a high heat transfercoefficient between a fluidised bed and an article immersed in it, it isdesirable to maintain a freely bubbling condition, such that the rapidand continuous movement of the particles can give rise to transfer ofheat between the article and the bulk of the bed. This, it was thought,would not occur in a quiescent bed where the particle movement is less.However it has now been found that unexpectedly high heat transfercoefficients are obtained between a hot glass article and a cooler bedof fluidised particulate material in a quiescent uniformly expandedstate and having selected characteristics.

It is found that there is thermal agitation of the uniformly fluidisedparticulate material at the hot glass surfaces when a hot glass sheet isquenched in the bed and there is greater rapidity of movement andturbulence of the fluidised particles in the region of the surfaces ofthe glass sheet than in the bulk of the bed. This results in a high rateof transfer of heat away from the glass surfaces. It is thought thatparticles which become heated by passing in proximity to the glasssurfaces then move rapidly away from the glass sheet and lose heat tothe fluidising air in the bulk of the bed.

A preferred method according to the invention includes regulating thegas flow to maintain said quiescent state of the fluidised bed bycreating a high pressure drop in the fluidising gas flow across amembrane through which fluidising gas enters the bed.

Further according to the invention the particulate material may compriseparticles of density in the range 0.3 g/cm³ to 3.97 g/cm³ and meanparticle size in the range 5μm to 120μm, the material being selected soas to be fluidised in said uniform quiescent state by fluidising gasflowing uniformly in the bed at a velocity in the range 0.045 cm/s to5.61 cm/s.

The density of the particles and their mean particle sizes are bothimportant in determining the suitability of a particulate material forconstituting the fluidised bed in a quiescent uniformly expanded stateemployed in the method of the invention. Generally an appropriateparticulate material for fluidisation in a quiescent uniformly expandedstate by fluidising air, when the bed is operating in ambient conditionsof normal room temperature and pressure, is one for which the numericalproduct of the particle density, in g/cm³, and the mean particle sizeinμm, does not exceed about 220.

The degree of toughening of a glass sheet which is achieved by themethod of the invention depends on the heat transfer coefficient betweenthe fluidised particulate material and the hot glass sheet immersed init. As already described there is thermal agitation on the hot glasssurfaces which give rise to transfer of heat rapidly away from thosesurfaces. However the properties of the particles themselves also affectthe magnitude of the heat transfer coefficient.

For thermally toughening flat soda-lime-silica glass of thickness in therange 2.3 mm to 12 mm, the method of the invention may comprise heatingthe glass to a temperature in the range 610° C. to 680° C., immersingthe glass in a fluidised bed in said quiescent state which has a thermalcapacity per unit volume at minimum fluidisation in the range 0.02cal/cm³ ° C. to 0.37 cal/cm³ ° C., and maintaining the fluidised bed ata temperature up to 150° C. to induce in the glass an average centraltensile stress in the range 22 MN/m² to 115 MN/m².

The maximum magnitude of average central tensile stress which can beachieved varies with the thickness of the glass and the heat transfercoefficient. By selection of a suitable material the heat transfercoefficient can be made high enough to produce toughened glass sheetshaving a central tensile stress as high as 40MN/m² in glass 2 mm thick,a central tensile stress as high as 50MN/m² in glass 3.0 mm thick, and acentral tensile stress as high as 104 MN/m² in glass which is 12 mmthickness. However, even higher central tensile stresses than these havebeen achieved as is shown in some of the Examples.

The particles may be a non-porous powdered α alumina of mean particlesize in the range 23μm to 54μm, and particle density 3.97 g/cm³, thethermal capacity per unit volume of the bed at minimum fluidisationbeing 0.32 cal/cm³ ° C.

The invention also comprehends a fluidised bed for use as a quenchingmedium for thermally toughening a hot glass sheet, comprising particlesof mean particle size in the range 5μm to 120 μm and having a particledensity in the range 0.3 g/cm³ to 3.97 g/cm³, and wherein the particlesare so selected that the bed is in a quiescent uniformly expanded stateof particulate fluidisation and has a thermal capacity per unit volumeof bed at minimum fluidisation in the range 0.02 cal/cm³ ° C. to 0.37cal/cm³ ° C.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention will now be described, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1 illustrates diagrammatically a vertical section through apparatusfor carrying out the method of the invention,

FIG. 2 is a detail in section of part of FIG. 1, and

FIG. 3 is a graph which illustrates a characteristic of a gas-fluidisedbed in a quiescent uniformly expanded state of particulate fluidisation,which is employed in carrying out the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, a vertical toughening ovenindicated generally at 1 has side walls 2 and a roof 3. The side walls 2and the roof 3 are made of the usual refractory material and the bottomof the oven is open, being defined by an elongated aperature 4 in abaseplate 5 on which the oven 1 is supported. A movable shutter, notshown, is provided in known manner to close the aperture 4.

A sheet of glass 6 to be bent and subsequently thermally toughened issuspended in the oven 1 by tongs 7 which engage the upper margin of thesheet 6 and are held closed in customary manner by the weight of theglass sheet gripped between the tong points. The tongs 7 are suspendedfrom a tong bar 8 which is suspended from a conventional hoist, notshown, and which runs on vertical guide rails 9 which extend downwardlyfrom the oven to guide the lowering and raising of the tong bar 8.

A pair of bending dies 10 and 11 are located on either side of the pathof the glass sheet 6 in a chamber 12, which is heated by hot gas flowsthrough ducts 12a. The interior of the chamber 12 and the dies 10 and 11are maintained at the same temperature as the temperature of the hotglass sheet 6 as it enters the chamber 12.

The die 10 is a solid male die mounted on a ram 13 and has a curvedfront face which defines the curvature to be imposed on the hot glasssheet. The die 11 is a ring frame female die carried by struts 14mounted on a backing plate 15 which is mounted on a ram 16. Thecurvature of the die frame 11 matches the curvature of the face of themale die 10.

The guide rails 9 extend downwardly through the chamber 12 to eitherside of the bending dies towards a container for a gas-fluidised bed 17of particulate refractory material in which the hot bent glass sheet isto be quenched. The container for the fluidised bed comprises anopen-topped rectangular tank 18 which is mounted on a scissors-liftplatform 19. When the platform 19 is in its raised position the top edgeof the tank 18 is just below the bending dies 10 and 11.

A micro-porous membrane 20, which is described in greater detail withreference to FIG. 2, extends across the base of the tank 18. The edgesof the membrane 20 are fixed between a flange 21 on the tank and aflange 22 on a plenum chamber 23 which forms the base of the tank. Theflanges and the edges of the plate 20 are bolted together as indicatedat 24. A gas inlet duct 25 is connected to the plenum chamber andfluidising air is supplied to the duct 25 at a regulated pressure. Themembrane is so constructed that fluidising air flows uniformly into thefluidised bed over the whole base of the bed to maintain the bed in aquiescent uniformly expanded state of particulate fluidisation.

Particulate refractory material in the tank 18 is maintained in thequiescent uniformly expanded state of particulate fluidisation by theupward flow of air uniformly distributed by the porous membrane 20. Theexpanded bed is in a substantially bubble-free quiescent state and thereare no regions of the bed which are not fluidised.

A preferred construction of micro-porous membrane is shown in FIG. 2 andis described in United Kingdom patent application No. 24124/76. Thismembrane comprises a steel plate 26 which has a regular distribution ofholes 27. The margins of the plate are drilled to provide passages forbolts 24. A gasket 28 is located between the lower face of the marginsof the plate and the flange 22 on the plenum chamber.

A number of layer 29 of strong micro-porous paper are laid on the plate26. For example fifteen sheets of paper may be used. The membrane iscompleted with a woven wire mesh 30, for example stainless steel meshwhich is laid on top of the paper. An upper gasket 31 is located betweenthe margins of the wire mesh 30 and the flange 21 on the tank.

A basket for catching cullet may be located near the plate 20, and isdesigned so as not to interfere with the uniform flow of fluidising airupwardly from the membrane.

Referring again to FIG. 1, the guide rails 9 extend downwardly to aposition below the bending dies and terminate in the region of the upperedge of the tank 18. A fixed frame indicated at 32 is mounted in thetank 18 and has upturned feet 33 at its base to receive the lower edgeof a glass sheet lowered into the fluidised bed when the tong bar 8 islowered beyond the bending dies by the hoist.

With the scissors-lift table 19 lowered and the tongs 7 and tong bar 8in their lowermost position at the bottom of the guides 9, a cool glasssheet to be bent and toughened is loaded onto the tongs. The hoist whenraises the suspended glass sheet into the oven 1 which is maintained ata temperature, for example 850° C., when toughening soda-lime-silicaglass. The glass sheet is rapidly heated to a temperature near itssoftening point for example a temperature in the range 610° C. to 680°C.

When the glass sheet has reached a required temperature uniformly, theshutter closing the aperture 4 is opened and the hot glass sheet islowered by the hoist into position between the open bending dies 10 and11. The rams 13 and 16 are operated and the dies close to bend thesheet. When the required curvature has been imparted to the sheet thedies open and the hot bent glass sheet is rapidly lowered into thefluidised bed in the tank 18 which has been raised to quenching positionby operation of the scissors-lift table 19 while the glass sheet wasbeing heated in the oven 1.

When high quality laminated glass products are to be producedincorporating thermally toughened glass sheets produced by quenching ina fluidised bed an improvement in optical quality has been observed whenthe surfaces of the glass sheet are subjected to a preliminary aircooling just before the glass is immersed in the fluidised bed. This maybe achieved by locating just above the upper edge of the tank 18 shallowblowing frames which direct cooling air onto the surfaces of the bentglass sheet as it leaves the bending dies and enters the fluidised bed.

The preliminary surface cooling is effective to "set-up" the surfaces ofthe glass sheet and thereby avoid minute variations in those surfacessuch as have sometimes been observed and which may be due to the thermalagitation of the fluidised particulate material on the glass surfaces.Such preliminary surface cooling would however only usually be employedwhen the glass is being used for the production of laminates of highoptical quality.

The fluidised bed is maintained at a suitable temperature for inducing arequired central tensile stress in the glass, for example 30° C. to 150°C., by the water cooling jackets 34 on the flat longer walls of the tank18, and by controlling the temperature of the fluidising air supplied tothe plenum chamber 23. The jackets 34 act as a heat sink which absorbsheat transferred through the bed from the hot glass sheet.

The lower edge of the hot glass sheet is uniformly chilled along itswhole length as it enters the horizontal quiescent surface of theexpanded fluidised bed so that there is no possibility of differenttensile stresses being generated in different areas of the surface ofthat edge of the glass, such as could lead to fracture. During itsdescent into the bed the lower edge always contacts fluidised materialin a quiescent uniformly expanded state of particulate fluidisation, andthis uniform treatment of the lower edge, regardless of upward flow ofparticulate material which may be generated on the hot glass surfacesimmediately they enter the fluidised bed, largely obviates fracture andthe problems of dealing with glass fragments in the bed. This togetherwith the avoidance of losses of glass sheets due to change of shape ofthe glass sheets and/or damage to the surface quality, ensures acommercially viable yield of toughened glasses.

Localised thermal agitation of the fluidised bed takes place on the hotglass surfaces, perhaps by rapid gas expansion in a manner akin to theboiling of a liquid. The agitation ensures that there is adequate heattransfer away from the glass surfaces into the bulk of the fluidisedbed, for example heat transfer coefficients between the bed and theglass sheet in the range of 0.003 cal/cm² ° C. sec to 0.02 cal/cm² ° C.sec are obtained. The heat transfer continues until well after the glasshas cooled below its strain point, with sufficient severity to ensurethat the centre-to-surface temperature gradients are maintained as theglass cools through its strain point, and the toughening stresses aredeveloped thereafter during the continuous cooling of the glass while itis still immersed in the bed.

The agitation of the fluidised material at the glass surfaces sets upcurrents in the bulk of the bed which ensure continual dissipation tothe remoter parts of the bed of the heat which is extracted from theglass by the thermal agitation of the bed in the region immediatelysurrounding the glass sheet. The water cooling jackets 34, acting as aheat sink, keep those remoter parts of the bed cool.

The sheet engages the feet 33 of the frame 32 at the bottom of itsdescent, thereby releasing the tongs 8. The glass sheet then rests onthe frame 32 while the glass sheet cools in the fluidised bed. The glasssheet remains in the fluidised bed until it is cooled sufficiently to behandled and the tank 18 is lowered by lowering the scissors-liftplatform to expose the fixed frame 32 and the supported toughened glasssheet which is then removed for subsequent cooling to room temperature.

The nature of the quiescent uniformly expanded state of particulatefluidisation of the fluidised bed is illustrated in FIG. 3 which is aplot of plenum pressure, that is, the pressure in the plenum chamber,against the height of the bed in the tank 18 using γ alumina particlesas described in Example 2, set out below, and with the tank size andfluidisation conditions of Example 2, and the temperature of the bed at80° C.

When the plenum pressure reached 15 kN/m² expansion of the bed began,the velocity of the fluidizing air through the bed then being sufficientto produce incipient fluidisation. That is, at this lower limit of gasvelocity the γ alumina particles just become suspended in the upwardlyflowing air.

Because of the use of a high pressure drop and a uniformly micro-porousmembrane of the kind illustrated in FIG. 2, in which the pressure dropacross the membrane is in excess of 60% of the plenum pressure, there isuniform distribution of fluidizing air flowing upwardly from the upperface of the membrane. This high pressure drop across the membrane makespossible sensitive regulation of the velocity of gas flow upwardlythrough the particulate material, thereby permitting regulation of thestate of quiescent fluidisation of the γ alumina between the minimumfluidisation state just described and a state of maximum expansion ofthe bed in which dense-phase fluidisation is maintained.

This sensitive regulation of the gas velocity is achieved by regulationof the plenum pressure in the chamber 23, and as the plenum pressureincreases there is no sudden or discontinuous change in the state of thebed. Rather the quiescent uniformly expanded state of the bed persists,as illustrated in FIG. 3, as the plenum pressure is increased to about25 kN/m² and the bed expands to a height of about 102 cm in the tank.

At this plenum pressure the first clearly recognisable bubble, forexample about 5 mm in diameter, may be observed breaking the surface ofthe quiescent bed, and this velocity of the fluidising air may beconsidered as the minimum bubbling velocity.

Because of the use of the high-pressure drop membrane 20, it has beenpossible to observe that this minimum bubbling velocity is notnecessarily the gas velocity producing maximum expansion of the bed, andfurther regulation of the plenum pressure up to 27 kN/m² produced amaximum bed height of 105 cm. While this increase in plenum pressure upto 27 kN/m² was effected more small bubbles were observed to break thebed surface, but the small random bubbles were not so significant as toadversely affect the capacity of the bed for quenching hot sheets ofglass, in particular thicker sheets of glass.

With increase of the plenum pressure beyond 27 kN/m², persistantbubbling of the bed occured and a tendency of the bed to collapse to aheight below its maximum height of 105 cm, was observed. In this statethe bed was unsuitable for toughening hot glass sheets.

In this example therefore the uniform quiescent expanded state of thefluidised bed of γ alumina, which was effective for toughening hot glasssheets is represented by the region of the curve of FIG. 3 lying betweenplenum pressures of 15 kN/m² and 27 kN/m², in which region sensitivecontrol of the state of fluidisation was possible, with consequentialcontrol of the uniform toughening stresses induced in the glass.

The effective heat transfer coefficient of the fluidised bed relative tothe hot glass is determined by the properties of the fluidising gas,usually air, the gas velocity in the bed, the properties of theparticulate refractory material notably the range of sizes of theparticles, the mean particle size, the density of the particles and, inthe case when the particles contain cavities, that is have a certainporosity or hollow structure, the density of the material of theparticles. The heat transfer coefficient also depends on thetemperatures of the glass and the bed, since if there is only a smalldifference between these temperatures, there will be little agitation onthe surface of the glass and the effective heat transfer coefficientwill be comparatively low.

Other factors affecting the heat transfer coefficient are the specificheat of the particles, and their average heat capacity. In each of thefollowing examples the numerical value of the product of the particledensity, in g/cm³, and the mean particle size in μm, is less than 220.This is a criterion which may be used for assessing the suitability of aparticulate material, that is its capability of being fluidised by airin a quiescent uniformly expanded state of particulate fluidisation,operating with ambient conditions of normal temperature and pressure.

Some examples of toughening of glass sheets of thickness in the range2.3 mm to 12 mm, using apparatus as in FIGS. 1 and 2, and a uniformquiescent expanded bed are set out below. In each of the followingExamples 1 to 11 the edges of the glass sheet are finished by beingrounded using a fine diamond grit wheel.

EXAMPLE 1

The particulate refractory is a γ form of porous alumina the propertiesof which are as follows:

    ______________________________________                                        Mean particle size (α)                                                                     =       64μm                                            Particle size range                                                                              =       20 to 160μm                                     particle density (ρ)                                                                         =       2.2 g/cm.sup.3                                     Material density   =       3.97 g/cm.sup.3                                    ρ×α                                                                              =       141                                                Material specific heat                                                                           =       0.2 cal/g° C.                               Thermal capacity per unit                                                     volume of bed at minimum                                                      fluidisation       =       0.21 cal/cm.sup.3 ° C.                      Velocity of fluidising air                                                    in bed             =       0.54 cm/s                                          ______________________________________                                    

With the bed maintained at 40° C. the degree of toughening of glasssheets of thickness in the range 2.3 mm to 12 mm with an initial glasstemperature in the range 610° C. to 670° C. was as follows:

    ______________________________________                                        Initial Glass              Average Central                                    Temperature  Glass Thickness                                                                             Tensile Stress                                     (° C.)                                                                              (mm)          (MN/m.sup.2)                                       ______________________________________                                        610          2.3           37                                                 610          10            92                                                 610          12            93.5                                               630          2.3           42.5                                               630          6             72.5                                               630          12            96                                                 650          2.3           46                                                 650          4             64                                                 650          6             75.7                                               650          8             92.7                                               650          10            96                                                 650          12            99                                                 670          2.3           44                                                 670          6             75                                                 670          10            100                                                ______________________________________                                    

The effective heat transfer coefficient between the bed and the glasssheets lay in the range 0.01 cal/cm² ° C. sec to 0.012 cal.cm² ° C. sec.

EXAMPLE 2

In a particular production run using the same γ form of porous aluminaas in Example 1, bent sheets of glass 2.3 mm thick were toughened. Thesesheets were subsequently used as a component of a laminated windscreenfor automobiles.

The properties of the γ alumina are as follows:

    ______________________________________                                        Mean particle size α                                                                     =      64 μm                                              Particle size range                                                                            =      30 to 150μm                                        Particle density (ρ)                                                                       =      2.2 g/cm.sup.3                                        Material density =      3.9 g/cm.sup.3                                        ρ×α                                                                            =      141                                                   Size of tank holding    38 cm × 215cm ×                           fluidised bed    =      105 cm deep                                           Plenum pressure  =      24 kN/m.sup.2                                         Pressure drop across membrane                                                                  =      15 kN/m.sup.2                                         Pressure drop across membrane                                                                  =      60% of plenum                                                                 pressure                                              Rate of flow of fluidising air                                                                 =      0.175 m.sup.3 /min                                    Velocity of fluidising air in                                                 bed              =      0.36 cm/s                                             Temperature of fluidised bed                                                                   =      60° C.                                         Temperature of glass: top edge                                                                 =      650° C. to 655° C.                      bottom edge      =      670° C. to 675° C.                      Resulting uniform central                                                     tensile stress in glass                                                                        =      38MN/m.sup.2 to 40MN/m.sup.2                          ______________________________________                                    

The effective heat transfer coefficient between the bed and the glasssheets lay in the range 0.01 cal/cm² ° C. sec to 0.012 cal/cm² ° C. sec.

EXAMPLE 3

In another production run, sheets of glass intended as components oflaminated aircraft windscreens and of thickness 3 mm, 4 mm, 6 mm, 8 mm,and 10 mm, were toughened in a uniform quiescent expanded fluidised bedof γ alumina. The same γ form of porous alumina was used as in Examples1 and 2.

    ______________________________________                                        Size of tank holding                                                          fluidised bed =      45cm × 345cm × 150cm deep                    Plenum pressure                                                                             =      30 kN/m.sup.2                                            Pressure drop across                                                          membrane      =      19.5 kN/m.sup.2                                          Pressure drop across                                                          membrane      =      65% of plenum pressure                                   Rate of flow of                                                               fluidising air                                                                              =      0.34 m.sup.3 /min                                        Velocity of fluidising                                                        air in bed    =      0.51 cm/s                                                Temperature of fluidised                                                      bed           =      60° C.                                            Temperature of glass                                                                        =      645° C. to 650° C.                         ______________________________________                                    

Resulting uniform central tensile stress induced in the glass was asfollows:

    ______________________________________                                        Thickness            Central Tensile Stress                                   ______________________________________                                        3.0 mm               48 MN/m.sup.2                                            4.0 mm               53 MN/m.sup.2                                            10.0 mm              80 MN/m.sup.2                                            ______________________________________                                    

The effective heat transfer coefficient between the bed and the glasssheets lay in the range 0.01 cal/cm² ° C. sec to 0.012 cal/cm² ° C. sec.

EXAMPLE 4

The particulate refractory material is a porous powdered aluminosilicatematerial, each particle containing 13% by weight alumina and 86% silica.The powdered material has the following properties:

    ______________________________________                                        Particle size range                                                                              =       up to 150μm                                     Mean particle size (α)                                                                     =       60μm                                            Particle density (ρ)                                                                         =       1.22 g/cm.sup.3                                    Material density   =       2.3 g/cm.sup.3                                     ρ×α                                                                              =       73                                                 Material specific heat                                                                           =       0.38 cal/g° C.                              Thermal capacity per unit                                                     volume of bed at minimum                                                      fluidisation       =       0.19 cal/cm.sup.3° C.                       Velocity of fluidising                                                        air in bed         =       0.21 cm/s                                          ______________________________________                                    

With the bed maintained at 40° C., the degree of toughening of glasssheets of thickness in the range 2.3 mm to 10 mm was as follows:

    ______________________________________                                        Initial Glass              Average Central                                    Temperature  Glass Thickness                                                                             Tensile Stress                                     (° C.)                                                                              (mm)          (MN/m.sup.2)                                       ______________________________________                                        650          2.3           30.8                                               650          4             44                                                 650          6             62.3                                               650          8             73                                                 650          10            79                                                 ______________________________________                                    

The effective heat transfer coefficient between the bed and the glasssheets lay in the range 0.007 cal/cm² ° C. sec to 0.009 cal/cm² ° C.sec.

EXAMPLE 5

Another form of a porous powdered composite aluminosilicate material wasused. Each particle is porous and contains 29% by weight alumina and 69%silica. This porous powder has the following properties:

    ______________________________________                                        Particle size range                                                                              =       up to 150μm                                     Mean Particle size (α)                                                                     =       75μm                                            Particle density (ρ)                                                                         =       1.21 g/cm.sup.3                                    ρ×α                                                                              =       91                                                 Material density   =       2.3 g/cm.sup.3                                     Material specific heat                                                                           =       0.2 cal/g° C.                               Thermal capacity per unit                                                     volume of bed at minimum                                                      fluidisation       =       0.11 cal/cm.sup.3 ° C.                      Velocity of fluidising air                                                    in bed             =       0.33 cm/s                                          ______________________________________                                    

With the bed maintained at 40° C., and the initial glass temperature inthe range 610° C. to 670° C., the degree of toughening of glass sheetsof thickness in the range 2.3 mm to 10 mm was as follows:

    ______________________________________                                        Initial Glass              Average Central                                    Temperature  Glass Thickness                                                                             Tensile Stress                                     (° C.)                                                                              (mm)          (MN/m.sup.2)                                       ______________________________________                                        610          6             51                                                 610          10            74                                                 630          2.3           31.5                                               630          6             53                                                 650          2.3           33.7                                               650          4             48.3                                               650          6             56                                                 650          8             71.3                                               650          10            84                                                 670          2.3           32                                                 670          6             58                                                 670          10            81.5                                               ______________________________________                                    

The effective heat transfer coefficient between the bed and the glasssheets lay in the range 0.007 cal/cm² ° C. sec to 0.01 cal/cm² ° C. sec.

EXAMPLE 6

A "Fillite" powder, which comprises the hollow glass spheres derivedfrom pulverised fuel ash from power station boilers, was selected tohave the following properties:

    ______________________________________                                        Particulate size range                                                                           =       20 to 160 μm                                    Mean Particle size (α)                                                                     =       77 μm                                           Particle density (ρ)                                                                         =       0.38 g/cm.sup.3                                    ρ×α                                                                              =       29                                                 Material density   =       2.6 g/cm.sup.3                                     Material specific heat                                                                           =       0.18 cal/g° C.                              Thermal capacity per unit                                                     volume of bed at minimum                                                      fluidisation       =       0.05 cal/cm.sup.3 ° C.                      The fluidisation velocity                                                     of the air in the                                                             Fillite            =       0.11 cm/s                                          ______________________________________                                    

The degree of toughening induced in the glass sheets which werethermally toughened in this fluidised bed can be represented by anaverage central tensile stress which was measured in conventional mannerand the results achieved for a range of glass thickness from 4 mm to 12mm, with different initial glass temperatures in the range 610° C. to670° C. and with the temperature of the fluidised bed at 40° C. are asfollows:

    ______________________________________                                        Initial Glass              Average Central                                    Temperature  Glass Thickness                                                                             Tensile Stress                                     (° C.)                                                                              (mm)          (MN/m.sup.2)                                       ______________________________________                                        610          10            40                                                 610          12            41                                                 630           6            30                                                 630          12            45                                                 650           4            22.4                                               650           6            32                                                 650           8            37                                                 650          10            39                                                 650          12            48.5                                               670           6            35                                                 670          10            50                                                 ______________________________________                                    

The effective heat transfer coefficient between the bed and the glasssheets lay in the range 0.003 cal/cm² ° C. sec to 0.004 cal/cm² ° C.sec.

EXAMPLE 7

Another grade of "Fillite" material was used having the followingproperties:

    ______________________________________                                        Mean particle size (α)                                                                     =       120μm                                           Particle density (ρ)                                                                         =       0.38 g/cm.sup.3                                    Material density   =       2.6 g/cm.sup.3                                     ρ×α                                                                              =       45                                                 Material specific heat                                                                           =       0.18 cal/g° C.                              Thermal capacity per unit                                                     volume of bed at   =       0.06 cal/cm.sup.3 ° C.                      minimum fluidisation                                                          Velocity of fluidising                                                        air in bed         =       0.27 cm/s                                          ______________________________________                                    

With initial glass temperatures in the range 630° C. to 670° C. and withthe bed at about 40° C., stresses induced in glass sheets of thickness 6mm to 10 mm were as follows:

    ______________________________________                                        Initial Glass              Average Central                                    Temperature  Glass Thickness                                                                             Tensile Stress                                     (° C.)                                                                              (mm)          (MN/m.sup.2)                                       ______________________________________                                        630          6             42                                                 630          8             49                                                 650          6             45.5                                               650          8             51                                                 650          10            63                                                 670          6             48                                                 670          8             53                                                 ______________________________________                                    

The effective heat transfer coefficient between the bed and the glasssheets lay in the range 0.005 to 0.006 cal/cm³ ° C. sec.

EXAMPLE 8

The particulate refractory material used was hollow carbon spheres ofthe kind known as "Carbospheres" having the following properties:

    ______________________________________                                        Particle size range                                                                              =       5 to 150μm                                      Mean particle size (α)                                                                     =       48μm                                            Particle density (ρ)                                                                         =       0.3 g/cm.sup.3                                     ρ×α                                                                              =       14.4                                               Material density   =       2.3 g/cm.sup.3                                     Material specific heat                                                                           =       0.123 cal/g° C.                             Thermal capacity per unit                                                     volume of bed at minimum                                                      fluidisation       =       0.02 cal/cm.sup.3 ° C.                      Velocity of fluidising air                                                    in bed             =       0.33 cm/s                                          ______________________________________                                    

The degree of toughening of glass sheets quenched in this fluidised bedmaintained at about 40° C. are as follows:

    ______________________________________                                        Initial Glass              Average Central                                    Temperature  Glass Thickness                                                                             Tensile Stress                                     (° C) (mm)          (MN/m.sup.2)                                       ______________________________________                                        610          10            44                                                 630          6             34                                                 650          4             26.3                                               650          6             32.7                                               650          8             40                                                 650          10            45                                                 670          6             36                                                 670          10            46                                                 ______________________________________                                    

The effective heat transfer coefficient between the bed and the glasssheets lay in the range 0.0035 cal/cm² ° C. sec to 0.004 cal/cm² ° C.sec.

EXAMPLE 9

The particulate refractory material was porous powdered nickel havingthe following properties:

    ______________________________________                                        Mean particle size (α)                                                                     =       5μm                                             Particle density (ρ)                                                                         =       2.35 g/cm.sup.3                                    Material density   =       8.9 g/cm.sup.3                                     ρ×α                                                                              =       12                                                 Material specific heat                                                                           =       0.106 cal/g° C.                             Thermal capacity per unit                                                     volume of bed minimum                                                         fluidisation state =       0.37 cal/cm.sup.3 ° C.                      Velocity of fluidising                                                        air in bed         =       0.045 cm/s                                         ______________________________________                                    

Glass sheets of thickness in the range 2.3 mm to 6 mm at an initialtemperature of 650° C. were quenched in a fluidised bed of this porousnickel powder which was in a quiescent state and was maintained at about40° C. The degree of toughening represented by average central tensilestress was as follows:

    ______________________________________                                                         Average Central Tensile                                      Glass Thickness  Stress                                                       (mm)             (MN/m.sup.2)                                                 ______________________________________                                        2.3              77                                                           3                95                                                           6                115                                                          ______________________________________                                    

The effective heat transfer coefficient between the bed and the glasssheets was 0.02 cal/cm² ° C. sec.

EXAMPLE 10

The particulate material was a non-porous powdered α alumina. A numberof α alumina materials of different mean particle size were used. Allthese materials had the following common properties:

    ______________________________________                                        Particle density (p)                                                                           =         3.97 g/cm.sup.3                                    Material density =         3.97 g/cm.sup.3                                    Material specific heat                                                                         =         0.2 cal/g° C.                               ______________________________________                                    

The α alumina material was available in different graded particle sizesof the material and four different fluidised beds were constituted asfollows:

    ______________________________________                                                                     Thermal                                                                Thermal                                                                              Capacity                                               Mean            Capacity                                                                             of minimum                                                                             Fluidising                              α Alum-                                                                       Particle        of     fluidised                                                                              gas                                     ina   Size(α)   Particle                                                                             bed      Velocity                                Bed   (μm)  ρ×α                                                                  (cal/° C.)                                                                    (cal/cm.sup.3 ° C.)                                                             (cm/s)                                  ______________________________________                                        A     23       92     5 × 10.sup.-9                                                                  0.32     1.02                                    B     29       116    10 × 10.sup.-9                                                                 0.32     1.62                                    C     45       180    38 × 10.sup.-9                                                                 0.32     3.90                                    D     54       216    66 × 10.sup.-9                                                                 0.32     5.61                                    ______________________________________                                    

Glass sheets of thickness in the range 2.3 mm to 12 mm were quenched inthese fluidised beds which are each at a temperature of 40° C. Theinitial temperature of the glass sheets was in the range 610° C. to 670°C. and the degree of toughening of the sheets is represented by anaverage central tensile stress in the range 42 MN/m² to 104 MN/m².

The effective heat transfer coefficient between the bed and the glasssheets was in the range 0.0062 cal/cm² ° C. sec to 0.0086 cal/cm² ° C.sec.

EXAMPLE 11

A bed of small solid glass spheres known as "Ballotini" was fluidised.Properties of the bed were as follows:

    ______________________________________                                        Particle size range =       0 to 75 μm                                     Mean particle size (α)                                                                      =       58 μm                                          Particle density (ρ)                                                                          =       2.5 g/cm.sup.3                                    ρ×α =       145                                               Thermal capacity per unit                                                     volume of bed at minimum                                                      fluidisation        =       0.34 cal/c.sup.3 ° C.                      Velocity of fluidising                                                        air in bed          =       0.41 cm/s                                         ______________________________________                                    

Sheets of glass of thickness in the range 2.3 mm to 10 mm were heated toan initial temperature in the range 630° C. to 670° C. and were quenchedin the fluidised bed which was maintained at a temperature of about 40°C.

The degree of toughening of the glass sheets was as follows:

    ______________________________________                                        Initial Glass              Average Central                                    Temperature  Glass Thickness                                                                             Tensile Stress                                     (° C.)                                                                              (mm)          (MN/m.sup.2)                                       ______________________________________                                        630          2.3           38                                                 630          6             72                                                 630          8             87                                                 650          2.3           40                                                 650          6             74.5                                               650          8             87                                                 650          10            90                                                 670          2.3           43                                                 670          6             80                                                 670          8             90                                                 ______________________________________                                    

The average effective heat transfer coefficient between the bed and theglass sheets was 0.011 cal/cm² ° C. sec.

To illustrate the high yield of unbroken and undistorted glass sheetsobtained when using a gas-fluidised bed according to the invention in aquiescent uniformly expanded state of particulate fluidisation, ascompared with the yield when using a bed in a bubbling state offluidisation, a number of similar sheets of glass of size 30 cm × 30 cmand of thickness 2 mm, 6 mm and 12 mm were treated. The glass sheets hadan edge finish in which the edges of the glass sheets were chamferedusing a bonded silicon carbide grinding wheel. This gave a rougher edgefinish than that of the glass sheets of Examples 1 to 11 which werefinished with a diamond grit wheel. The invention made a high yieldpossible even with this rougher, and cheaper, edge finish.

Each sheet was heated to a temperature as set out below and thenimmersed in a fluidised bed of the γ form of porous alumina described inExample 1.

For the purpose of these yield tests some hot glass sheets were immersedin a fluidised bed in a quiescent state as described in Example 1. Abubbling state of fluidisation of the bed was then produced byincreasing the fluidising gas velocity above the value producing maximumexpansion of the bed, and an equal number of hot glass sheets wereimmersed in the bubbling bed.

The yield of dimensionally acceptable unbroken glass sheets, as apercentage of the total number of sheets treated, was as follows:

    ______________________________________                                        Glass thickness = 2 mm                                                        Glass Temperature                                                                         Yield                                                             ° C. QUIESCENT BED BUBBLING BED                                        ______________________________________                                        645          95%          52%                                                 660         100%          80%                                                 ______________________________________                                    

    ______________________________________                                        Glass thickness = 6 mm                                                        Glass Temperature                                                                         Yield                                                             ° C. QUIESCENT BED BUBBLING BED                                        ______________________________________                                        640          80%          40%                                                 645         100%          60%                                                 ______________________________________                                    

    ______________________________________                                        Glass thickness - 12 mm                                                       Glass Temperature                                                                         Yield                                                             ° C. QUIESCENT RED BUBBLING RED                                        ______________________________________                                        635          80%          40%                                                 645         100%          75%                                                 ______________________________________                                    

Although the above examples were obtained using 30 cm × 30 cm squaresheets of glass even lower yields with respect to fracture anddistortion result when treating large sheets of glass such as of motorvehicle windscreen size in a bubbling fluidised bed. In contrast theyields obtained when treating such larger sheets of glass in a quiescentfluidised bed are at least as good as those of the examples referred toabove.

The value of the stresses induced in the glass decreases as the bedtemperature increases and in the limit, which may be about 300° C. orhigher, the stresses in the glass are such that the glass is annealedrather than toughened. Heating and/or cooling elements may be providedon the side walls of the tank 18 for controlling the temperature of thefluidised bed. In all the Examples the sheets of glass were commercialsoda-limesilica glass such as is used in the manufacture of aircraftwindscreen panels, automobile windscreens, ship's windows andarchitectural panels. Glass of other compositions can be toughened orannealed in the same way using the method of the invention. Alsoarticles other than glass sheets, for example pressed glass articlessuch as insulators or lens blanks, or blown glass articles can betoughened or annealed by the method of the invention.

A fluidised bed according to the invention may be used for other thermaltreatments of glass, for example for the heating of a relatively coldglass article prior to a further processing step, heat transfer from thefluidised material to the glass which is immersed in the bed beingfacilitated without damage to the glass, even when the glass hasattained a temperature at which it is susceptible to damage by irregularforces.

The invention may also be used for thermally toughening glass sheetswhich have been heated and bent while supported in a near-verticalposition, and advanced along a horizontal path, as described in U.S.Pat. No. 3,880,635. In the apparatus described in that application thebending dies are enclosed in a heated chamber which is tilted from aninclined position to a position in which the bent glass sheet betweenthe bending dies is vertical and can be lowered vertically into aquiescent fluidised bed of the kind described above.

In another process employing the invention a glass sheet may be heatedby immersing the sheet in a fluidised bed which is at a sufficientlyhigh temperature to heat the glass to pre-bending temperature. Afterremoval from the hot bed the sheet is bent, and the bent sheet is thentoughened by immersing the glass in a fluidised bed which is in aquiescent uniformly expanded state of particulate fluidisation asdescribed above. The glass sheet could be carried by the same set oftongs throughout the heating bending and toughening, the tongs beingadjustably mounted so that they move to follow the bent shape of theglass. In another arrangement each glass sheet is suspended fromnon-adjustable tongs for heating and is transferred to lower-edgesupport during bending in the manner described in U.S. Pat. No.3,880,635, the bent glass sheet being engaged by a second set of tongswhich are arranged according to the bent shape of the glass, and loweredinto the quiescent fluidised bed for quenching.

I claim:
 1. In a method of thermally treating glass in which the glassis contacted with a gas-fluidised particulate material to effect heattransfer between the surfaces of the glass and the fluidised material,and wherein gas is fed to said particulate material under pressure froman external source, the improvement wherein said gas-fluidisedparticulate material with which the glass is contacted is placed in aquiescent uniformly expanded state of particulate fluidisation throughcontrol of the gas feed such that the gas velocity while the glass isbeing contacted by said particulate material is between that velocitycorresponding to incipient fluidisation and that velocity correspondingto maximum expansion.
 2. In a method of thermally treating glass whereinthe glass is heated to a temperature above its strain point and isimmersed in a gas-fluidised bed of particulate material, and wherein thegas enters the bed from a plenum chamber, the improvement wherein, priorto said immersion, said gas-fluidised particulate material is placed ina quiescent uniformly expanded state of particulate fluidisation throughcontrol of the plenum pressure such that the gas velocity while theglass is being contacted by the bed is between that velocitycorresponding to incipient fluidisation and that velocity correspondingto maximum expansion of the bed.
 3. The improvement according to claim 2wherein the glass is a glass sheet that is thermally toughened byheating the sheet and then lowering the hot glass sheet into thequiescent uniformly expanded bed of particulate material.
 4. Theimprovement according to claim 3 wherein the bed is maintained at atemperature in the range 30° C. to 150° C.
 5. The improvement accordingto claim 1 wherein said gas-fluidised particulate material is maintainedin a quiescent uniformly expanded state of particulate fluidisationthrough regulation of the fluidising gas flow by creating a highpressure drop in the fluidising gas flow across a membrane through whichfluidising gas enters the bed.
 6. The improvement according to claim 1wherein said gas-fluidised particulate material comprises particles ofdensity in the range 0.3 g/cm³ to 3.97 g/cm³ and mean particle size inthe range 5 μm to 120 μm, the material being selected so as to befluidised in a quiescent uniformly expanded state of particulatefluidisation by fluidising gas flowing uniformly in the bed at avelocity in the range 0.045 cm/s to 5.61 cm/s.
 7. The improvementaccording to claim 6 for thermally toughening flat soda-lime-silicaglass of thickness in the range 2.3 mm to 12 mm wherein the glass isheated to a temperature in the range 610° C. to 680° C., the glass isimmersed in said fluidised bed in said quiescent state which has athermal capacity per unit volume at minimum fluidisation in the range0.02 cal/cm³ ° C. to 0.37 cal/cm³ ° C., and wherein the fluidised bed ismaintained at a temperature up to 150° C. to induce in the glass anaverage central tensile stress in the range 22 MN/m² to 115 MN/m². 8.The improvement according to claim 6 wherein the particles are anon-porous powdered α-alumina of mean particle size in the range 23 μmto 54 μm and particle density 3.97 g/cm³, the thermal capacity per unitvolume of the bed at minimum fluidisation being 0.32 cal/cm³ ° C.
 9. Ina method of thermally treating glass in which the glass is heated to atemperature above its strain point and is immersed in gas-fluidisedparticulate material to effect heat transfer between the surfaces of theglass and the fluidised material, the improvement wherein saidgas-fluidised particulate material in which the glass is immersed isplaced, prior to said immersion, in a quiescent uniformly expanded stateof particulate fluidisation through flow control of fluidising gas toengender a uniform distribution of fluidising gas in the particulatematerial at a gas flow velocity through the particulate material betweenthat velocity corresponding to incipient fluidisation and that velocitycorresponding to maximum expansion of the particulate material.
 10. In amethod of thermally treating glass in which the glass is contacted witha gas-fluidised particulate material to effect heat transfer between thesurfaces of the glass and the fluidised material, and wherein fluidisinggas is fed to said particulate material through a membrane from anexternal source, the improvement wherein said gas-fluidised particulatematerial with which the glass is contacted is placed in a quiescentuniformly expanded state of particulate fluidisation through control ofgas fed through the membrane to provide uniform distribution offluidising gas flow upwardly from the membrane at a gas velocity betweenthat velocity corresponding to incipient fluidisation and that velocitycorresponding to maximum expansion.
 11. The improvement according toclaim 10, wherein the gas fed is through the membrane from a plenumchamber, and the plenum pressure in the plenum chamber is regulated tocontrol the velocity of said upward fluidising gas flow from themembrane.
 12. The improvement according to claim 11, wherein regulationof the plenum pressure is effected by regulating the pressure of thesupply of fluidising gas to the plenum chamber.
 13. In a method ofthermally toughening glass in which the glass is heated to a temperatureabove its strain point and is immersed in a gas-fluidised bed ofparticulate material, and wherein the gas enters the bed through amembrane from a plenum chamber, the improvement wherein, prior to saidimmersion, said gas fluidised material is placed in a quiescentuniformly expanded state of particulate fluidisation by regulation ofthe plenum pressure in the plenum chamber such that the velocity of theflow of fluidising gas upwardly from the membrane is between thatvelocity corresponding to incipient fluidisation and that velocitycorresponding to maximum expansion.
 14. The improvement according toclaim 13, wherein regulation of the plenum pressure in the plenumchamber is effected by regulating the pressure of the supply offluidising gas to the plenum chamber.
 15. In a method of thermallytoughening a glass sheet wherein the glass sheet is heated to atemperature above its strain point and is lowered into a gas-fluidisedbed of particulate material, the improvement wherein said gas-fluidisedparticulate material of the bed is placed, prior to immersion of the hotglass in the bed, in a quiescent uniformly expanded state of particulatefluidisation with a quiescent surface by control of gas feed to the bedsuch that the gas velocity in the bed is between that velocitycorresponding to incipient fluidisation and that velocity correspondingto maximum expansion and the lower edge of the hot glass sheet isuniformly chilled as it enters the quiescent surface of the expandedbed.
 16. In a method of thermally toughening a glass sheet wherein theglass sheet is heated to a temperature above its strain point and islowered into a gas-fluidised bed of particulate material, theimprovement wherein said gas-fluidised bed is placed, prior to immersionof the hot glass in the bed, in a quiescent uniformly expanded state ofparticulate fluidisation by control of gas fed to the bed such that thegas velocity in the bed is at least that velocity at which the expandedbed has a horizontal quiescent surface which chills the lower edge ofthe hot glass sheet uniformly as the lower edge enters the horizontalquiescent surface.